1. Field of the Invention
The present invention relates to a signal-processing circuit and, more particularly, to a signal-processing circuit adapted for signal compression and expansion performed in a transmitter and in a receiver used in a mobile communication system using automobile telephony or other mobile telephony.
2. Description of the Prior Art
Where an analog signal such as an audio signal is modulated and transmitted by mobile communication equipment using automobile telephony or other mobile telephony, signal compression and expansion circuits are used to compress and transmit the signal. When the signal is received, it is expanded to reduce noise. One example of the structure of a baseband signal-processing circuit of a transmitter/receiver used in a mobile communication system employing these signal compression and expansion circuits is shown in FIG. 1.
Referring to FIG. 1, an audio signal is received from a microphone (mike) on the side of the transmitter. A requisite band is extracted by a bandpass filter 1. Then, the signal is compressed into a narrower range of amplitudes by a signal compressor circuit 2. The RF energy level of this compressed audio signal is increased by a pre-emphasis circuit 3. The output signal from this pre-emphasis circuit 3 is supplied to a signal synthesis circuit 6 via a deviation limiter 4 and via a splutter filter 5. The signal synthesis circuit 6 combines digitally coded data applied from a microcomputer (abbreviated micro) via a low-pass filter 7 with the audio signal, the digitally coded data indicating the used channel or other information. The output signal from the signal synthesis circuit 6 is supplied to a modulator circuit (not shown), where the signal is frequency-modulated. This frequency-modulated signal is then transmitted.
On the side of the receiver, the RF component signal level of the received analog signal is returned to the original level by a deemphasis circuit 8. Then, a requisite band is extracted by a bandpass filter 9, and the original signal prior to the compression is reconstructed by a signal expansion circuit 10 and supplied to an earphone, for example. The data component which is included in the received analog signal and indicates the used channel or other information is extracted by a low-pass filter 11. The output signal from this filter 11 is shaped into a rectangular wave by a wave-shaping circuit 12 and supplied to the microcomputer as digitally coded data indicating the used channel.
The input/output characteristics of the signal expansion circuit 10 used in the above-described mobile communication system are shown in FIG. 2. A standard stipulates that when the relative input changes by 1 dB, the relative output varies by 2 dB. The input/output characteristics of the signal compressor circuit 2 are exactly opposite to the input/output characteristics of the signal expansion circuit 10.
A conventional structure of this signal expansion circuit 10 is shown in FIG. 3. This circuit includes a level detector circuit 13 that detects the level of its analog input signal S.sub.i which is also applied to a gain control amplifier (GCA) 14. The gain of this amplifier 14 is controlled according to the output signal from the detector circuit 13. As a result, an expanded analog signal S.sub.0 is delivered from the amplifier 14. To provide a signal compressor circuit 2 instead, negative feedback is applied to the signal expansion circuit 10 of the structure described above.
Heretofore, ICs comprising bipolar devices have been widely used as the conventional signal compressor circuit 2 and as the signal expansion circuit 10 described above. Since limitations are imposed on the electric power supplied to an automobile telephone or other mobile telephone, the use of a device consuming only a small amount of electric power is preferable. Accordingly, there is an increasing demand for a signal compressor circuit 2 and a signal expansion circuit 10 which are fabricated from CMOS circuits. However, the present situation is that signal compressor circuits and signal expansion circuits which are made of CMOS circuits and have good input/output characteristics have not yet realized.